During combustion of solid fuel, in particular coal, certain impurities in the fuel, especially sulfur, have to be contended with. These impurities involve certain drawbacks, especially for the environment. One advantage of allowing the combustion to take place in a fluidized bed is that the bed material may consist of, or be supplied with, an absorbent material which absorbs and binds the impurities so that these are deposited in the form of harmless powder or slime.
Doolmite or limestone may be used as a bed material and sulfur absorbent material. Such material is supplied to the combustion chamber in particulate form, for example with a particle size in the range of from about 0.05 to about 6 mm. When sulfur-containing fuel is burnt in the fluidized bed, the sulfur reacts with the oxygen of the combustion air and forms sulfur dioxide which reacts with the active component or components in the sulfur absorbent material, for example calcium. This may occur, for example, by the absorbent material first being calcined, that is, carbon dioxide escapes from the absorbent material. After that, the surface of each particle of the sulfur absorbent material becomes sulfatized, with the formation of calcium sulfate on its surface. The sulfatization of the particles then becomes deeper and deeper from the surface.
It has been found that very small particles of the absorbent material are sulfatized very rapidly whereas large particles, even after a long residence time in the fluidized bed, are still sulfatized only to a minor extent.
The supply of absorbent material takes place continuously and is controlled with respect to the sulfur content in the fuel. Fine particles of the absorbent material are sulfatized rapidly and some of these leave the combustion chamber with the exhaust gases and are collected in a cleaner. Coarse particles are sulfatized slowly and remain in the bed. For the bed height to be maintained constant, bed material must be removed from the fluidized bed either constantly or intermittently. This withdrawn bed material is then often sulfatized only partially in spite of a long residence time in the bed.
The present invention aims to provide a plant for the combustion of impure solid fuel which makes it possible to improve the utilization of, and thus reduce the consumption of sulfur absorbent material.